In marine seismic exploration, an acoustic source disposed beneath the water surface generates a signal which is reflected from layers beneath the ocean floor and received by hydrophones connected to a remote recording unit. The received reflected signal provides information about the subsurface characteristics of the earth.
Conventional acoustic sources used in marine exploration consist of an array of many air guns or water guns fired simultaneously at preselected time intervals. The shape of the acoustic wave or "signature" of the array is dependent upon the depth at which the array is fired and on the synchronization of the firing instant among all the guns in the array. If one of the two parameters or both are not substantially the samefor all guns the seismic signature will be less than optimum. If the depths of the guns are different, the firing instant of each gun can be changed (by fractions of a millisecond) so that the resulting acoustic wavefront is at maximum amplitude and strength.
Heretofore, the depth of each gun was controlled by the towing speed, weight of the gun, and by surface buoys. With the advent of more sophisticated recording equipment providing better signal resolution, the quality of the acoustic signal needed to be refined. The exact depth of each gun in the array, and the exact firing instant of each gun needed to be determined.
First attempts at making the guns "fly" at the same depth were made by using various types of hydrofoils and "birds" similar to those used on marine streamer cables. However, the weight and shape of the guns did not lend themselves to that type of arrangement.
An early attempt to determine gun depth was made by placing the depth transducer aboard ship. The transducer detected hydrostatic pressure through a long open-ended tube having the open end adjacent the acoustic source. A regulated air flow through the tube kept the tube purged. A change in the flow rate through the tube was related to a change in hydrostatic head. The use of larger pneumatic acoustic sources required a greater air flow through the tube to keep it purged. The air flow increase reduced the sensitivity of the sensing system.
Later attempts were made to determine gun depth by placing depth transducers on each gun. The firing instant of each gun would be varied according to its depth. This arrangement worked for a short period of time until the shock wave generated by the sources disabled the sensors.
It is an object of this invention to provide a ruggedized depth sensor for mounting directly on a seismic source.